Elevated CO2 enhanced the incorporation of 13C-residue into plant but depressed it in the microbe in the tomato (Solanum lycopersicum L.) rhizosphere soils

The translation of plant residue carbon in plant–soil–microbe systems under atmospheric CO2 (aCO2) and elevated CO2 (eCO2) is key to understanding the response of ecosystems to climatic change. Using stable isotope probing technology, we conducted an experiment on 3.5 g·kg−1 tomato 13C-residue (1050 ‰) added to tomato soils to investigate the decomposition and incorporation of 13C-residue under varied CO2 concentrations (aCO2 and eCO2: 400 and 800 μmol·mol−1, respectively). The results showed that the 13C-residue decomposition rate was stimulated by approximately 7.2 % and the 13C amount in plant stems was stimulated by 15.6 % (22.2 vs. 19.2 μg/pot), while the total 13C amount in soil microbial phospholipid fatty acids (PLFAs) was significantly decreased by 16.3 % (541 vs. 646 nmol·g−1), the DNA δ13C in soil microbes decreased by 5.1 % (−27.629 ‰ vs. −26.289 ‰), and soil δ13C decreased by 7.1 % (49.9 ‰ vs. 53.7 ‰) under eCO2. The microbial community structure involved in 13C-residue incorporation was significantly affected by the CO2 concentration. Firmicutes (Caldalkalibacillus 20.4 % and Bacillus 20.4 %) and Actinobacteria (Streptomyces 6.6 %) were the dominant phyla under aCO2 and eCO2, respectively. Proteobacteria (Dyella) were the most depleted microbes under eCO2, whereas almost no genera were found under aCO2. The 13C-residue decomposition rate was positively correlated with soil NO3−-N (r = 0.998, P < 0.05) at t1 (17 days of cultivation under varied CO2 concentrations), and DNA δ13C was positively related with soil pH (r = 0.978, P < 0.05) at t2 (35 days of cultivation under varied CO2 concentrations). Mantel tests showed that the microbial community composition involved in 13C incorporation was significantly correlated with the soil pH under various CO2 conditions (r = 0.879, P < 0.05). These results indicate that eCO2 stimulated the decomposition of 13C-residue and the transportation of 13C to the plant stem, but suppressed the incorporation of 13C into microbes (PLFA and DNA), which were adjusted by the rhizosphere N status and soil pH.